Pulsed voltage surges resistant enamelled wires

ABSTRACT

A pulsed voltage surges resistant enamelled wire comprises a metal conductive wire and at least one shield layer outside the wire, the shield layer is provided by a coating composition comprising (a) a synthetic resin, (b) an organic solvent and (c) metal oxide particles, wherein the metal oxide particles comprise α-form Al 2 O 3  particles, γ-form Al 2 O 3  particles and transition metal oxide particles.

BACKGROUND OF INVENTION

[0001] It is known that conventional types of speed drives cannot meet the requirements of efficiency, exactness and cost because of their high installation cost, smaller torque at slow speed, high maintenance fee and high energy consumption. Though pulse width modulated (PWM) type of inverters can meet the aforementioned requirements, it has been found that the use of PWM inverters causes premature failure of enamelled wires because of the inverters' high peak voltage values, pulsed voltage surges and harmonics, boost up and down, and high switching frequencies. Specifically, pulsed voltage surges are formed within a very short time of the order of μsec, and this will cause the temperature to suddenly increase (i.e. the effect of pulse voltage surges on temperature is more significant than that of corona discharge). The sudden increase of temperature will cause the thermal-oxidation decomposition of the insulation coating layers of enamelled wires and shorten the lifetime of the wires.

[0002] U.S. Pat. No. 5,654,095 provides an approach to meet the aforementioned requirements and discloses a pulsed voltage surge resistant enamelled wire which can withstand voltage surges approaching 3000 volts and is resistant to a high temperature up to 300° C., wherein the arising rate of the voltage is higher than 100 kV/μsec and the frequency is less than 20 kHz. The enamelled wire of U.S. Pat. No. 5,654,095 is characterized by the addition of metal oxide particles with a particle size of from 0.05 to 1 micron to the shield layer of enamelled wire to provide the desired pulsed voltage surge resistance. According to the disclosure of U.S. Pat. No. 5,654,095, metal oxides which can effectively resist pulse voltage surges and increase the lifetime of enamelled wires include titanium dioxide, alumina, silica, zirconium oxide, zinc oxide, iron oxide and various naturally occurring clays such as in lines 57-59 of column (see column 4, lines 57-59). Though the examples of U.S. Pat. No. 5,654,095 illustrated the metal oxide as Al₂O₃, they were totally silent on the structure of Al₂O₃.

[0003] There are two major structures of Al₂O₃—α-form and γ-form. α-form is a trigonal (R-3CH) structure wherein the lattice constants a=b=4.8 □ and c=13.0 □, the lattice angles α=β=90° and γ=120°. γ-form is a cubic (Fd-3mS) structure wherein the lattice constants a=b=c=7.9 □ and the lattice angles α=β=γ=90°. The structure of α-form is more compact than that of γ-form. In other words, the structure of γ-form is closer to an amorphous phase and is significantly different from that of α-form.

[0004] It has been found that, in the shield layer(s) of an enamelled wire, the use of both α-form Al₂O₃ particles and γ-form Al₂O₃ particles can provide pulsed voltage surge resistance much better than that provided by α-form Al₂O₃ particles or γ-form Al₂O₃ particles alone. This combination has been disclosed in U.S. Pat. No. 6,136,434. U.S. Pat. No. 6,136,434 is incorporated here for reference.

[0005] The applicant discovered that, the use of α-form and γ-form Al₂O₃ particles in and particles of transition metal oxide such as Cr₂O₃ in the shield layer(s) of an enamelled wire, can provide pulsed voltage surge resistance superior to that provided by the combination of α-form Al₂O₃ particles and γ-form Al₂O₃ particles.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the invention to provide an enamelled wire which has increased resistance to insulation degradation caused by pulsed voltage surges.

[0007] In the broader aspects of the invention, there is provided a pulsed voltage surge resistant enamelled wire comprising a metal wire and at least one pulsed voltage surge shield layer overlaying the metal wire, wherein the shield layer is provided by at least one polymer containing metal oxide particles dispersed therein, and wherein the metal oxide particles comprise α-form and γ-form Al₂O₃ particles and transition metal oxide particles. The shield layer containing α-form and γ-form Al₂O₃ particles and transition metal oxide particles can render the enamelled wire to be resistant to pulsed voltage surges without impairing the other properties of the enamelled wire.

[0008] These and other objects, advantages and features of the present invention will be more fully understood and appreciated by reference to the written specification.

DETAILED DESCRIPTION OF THE INVENTION

[0009] While this specification concludes with claims particularly pointing out and distinctly claiming that which is considered to be the invention, it is believed that the invention can be better understood from a reading of the following detailed description of the invention.

[0010] Accordingly, the present invention provides an enamelled wire which comprises a metal wire and at least one coating layer outside the wire, wherein the components of the at least one coating layer can be identical or different with the proviso that at least one of the coating layer contains α-form and γ-form Al₂O₃ particles and transition metal oxide particles. In other words, in case the enamelled wire comprises a single outside coating layer, the single outside coating layer is the shield layer which contains α-form and γ-form Al₂O₃ particles and transition metal oxide particles; otherwise, at least one of the outside coating layers is the shield layer which contains α-form and γ-form Al₂O₃ particles and transition metal oxide particles.

[0011] The metal wire of the present invention can be in any form, generally is in circular or rectangular form. As in circular form, it is preferred that the diameter of the wire is from 0.05 to 3.2 mm, more preferred from 0.10 to 1.5 mm, and most preferred from 0.35 to 1.2 mm.

[0012] Each coating layer of the present invention is provided by a coating composition comprising (a) a synthetic resin and (b) an organic solvent. The synthetic resin and organic solvent for the each coating layer can be identical or different. The coating composition may optionally comprise other conventional components suitable for coating layers of an enamelled wire such as dyes, pigments, dispersants, and the likes. The selected optional components and their amounts should not affect the desired properties of the coating layer. Any synthetic resins conventionally used in enamelled wires can be used in the coating composition. The synthetic resins used in the present invention can be, but not limited to, modified or unmodified polyacetal, polyurethane, polyester, polyesterimide, polyamideimide, polyamide, polysulfone, polyimide resins, or mixtures thereof. The selection of synthetic resin depends on the required temperature resistance and insulation properties on the coating layers. Furthermore, persons skilled in the art can choose an organic solvent suitable to the selected synthetic resin. The organic solvent can be, but not limited to, cresols, hydrocarbons, dimethyl phenol, toluene, xylene, ethyl benzene, N,N-dimethyl formamide (DMF), N-methyl-pyrrolidone (NMP), esters, ketones, or mixtures thereof. The combination of the synthetic resin and the organic solvent is, based on the total weight of the synthetic resin and the organic solvent, from 20 to 80 wt % of synthetic resin and from 20 to 80 wt % of organic solvent, and more preferably from 25 to 75 wt % synthetic resin and from 75 to 25 wt % organic solvent.

[0013] In order to provide the desired pulsed voltage surges resistance, the enamelled wire of the present invention comprises at least outside coating layer which is a shield layer and provided by a coating containing metal oxide particles, wherein the metal oxide particles comprise α-form and γ-form Al₂O₃ particles and transition metal oxide particles. Examples of the transition metal oxide are Cr₂O₃, TiO₂, SiO₂, Zr₂O₃, ZnO and Fe₂O₃, and Cr₂O₃ is preferred. The particle size of the metal oxide particles suitable for the present invention is from 0.001 to 10 microns, preferably from 0.01 to 5 microns, and more preferably from 0.05 to 1.0 micron. It is preferred that the total amount of the metal oxide particles in a shield layer is, based on 100 parts by weight of synthetic resin, from 3 to 20 parts by weight (3 to 20 PHR), and from 5 to 15 parts by weight (5 to 15 PHR) is more preferred. The amount of the transition metal oxide particles in a shield layer is, based on 100 parts by weight of synthetic resin, 2 to 19 parts by weight (2 to 19 PHR), from 5 to 15 parts by weight (5 to 15 PHR) is preferred, and from 5 to 10 parts by weight (PHR) is more preferred. And the amount ratio between the α-form Al₂O₃ particles and γ-form Al₂O₃ particles is from 1:1 to 1:100, from 1:5 to 1:50 is preferred, and from 1:5 to 1:15 is more preferred. The metal oxide particles can be uniformly dispersed into the coating composition by high shear mixing or with the use of a mixing apparatus. Optionally, a dispersant can be used to facilitate the dispersion of the metal oxide particles and prevent the particles from precipitation. The amount of dispersant, if used, is from 0.01 to 2 parts by weight per hundred parts by weight of synthetic resin and organic solvent.

[0014] Each of the coating layer(s) of an enamelled wire is provided by applying a corresponding coating composition on the metal wire, and drying and curing the coating composition. Generally, the thickness of each layer is from 2.0 to 5.0 mils and the layer is provided by repeatedly applying the coating composition on the surface of the wire for five (5) to fifteen (15) passages. The method of applying the coating depends on the viscosity of the coating composition. Generally, at 30° C., a coating composition having a viscosity higher than 500 cps is applied by dies, a coating composition having a viscosity of from 100 to 200 cps is applied by a roller and a coating composition having a viscosity of from 40 to 100 cps is applied by felt. The speed for applying the coating composition is between 3 and 450 m/min. The coated wire, after each coating layer has been applied, is fed into a furnace to dry and cure the layer. The temperature of furnace will depend on the species of coating, the length of furnace and the thickness of coating layer. Generally, the temperature at the inlet of the furnace is between 300 and 350° C. and the temperature at the outlet of the furnace is between 350 and 700° C.

[0015] The following examples are offered by way of illustration. In the examples, the formulations of coatings and Al₂O₃ particles applied are as follows:

[0016] (1) PAI coating: polyamideimide coating, available from Tai-I Electric Wire & Cable Co,. Ltd. ROC. as TAI-AIW-31.5, which can be cured by heating at an elevated temperature, the solvent of the coating comprises xylene, NMP and DMF, viscosity: 1500 cps/30° C., solid content: 30.2%.

[0017] (2) PEI coating: polyesterimide coating, available from Nisshoku-Schenectady Kagaku Co. Ltd. Japan as ISOMID-42, which can be cured by heating at an elevated temperature and through a transesterification or esterification reaction, the solvent of the coating comprises xylene, hydrocarbons, cresols and phenol, viscosity: 2050 cps/30° C., solid contents: 42.2%.

[0018] (3) Al₂O₃ particles: α-form particles and γ-form particles, particle size of α-form=about 0.3 micron, particle size of γ-form=about 0.05 micron.

[0019] (4) Cr₂O₃ particles: particle size=about 0.6 micron

EXAMPLES Comparative Example C1

[0020] PEI and PAI coatings were separately applied by dies onto the surface of copper wires having a diameter of 1.024 mm under the following conditions:

[0021] (i) inner coating layer (the coating layer directly attached to the copper wire):

[0022] coating: PEI coating

[0023] coating passages: nine (9) passages

[0024] linear coating speed: 9 m/min

[0025] (ii) outer coating layer (the coating layer overlapping the inner coating layer):

[0026] coating: PAI coating

[0027] coating passages: three (3) passages

[0028] linear coating speed: 9 m/min

[0029] (iii) furnace: length=3.5 m, inlet temperature=360° C., outlet temperature=480° C.

[0030] The properties of the coated wires are shown in Table I.

Comparative Example C2

[0031] The same as Comparative Example C1 with the exception that 5-10% α-form Al₂O₃ particles, based on the weight of synthetic resin, were added to the PAI coating and the Al₂O₃ particle-containing PAI coatings were mixed at high stirring speed. The properties of the coated wires are shown in Table I.

Comparative Example C3

[0032] The same as Comparative Example C1 with the exception that 5-10% γ-form Al₂O₃ particles, based on the weight of synthetic resin, were added to the PAI coating and the Al₂O₃ particle-containing PAI coatings were mixed at high stirring speed. The properties of the coated wires are shown in Table I.

Comparative Example C4

[0033] The same as Comparative Example 1 with the exception that 5-10% Al₂O₃ particles (α-form/γ-form-={fraction (1/9)}), based on the weight of synthetic resin, were added to the PAI coating and the Al₂O₃ particle-containing PAI coatings were mixed at high stirring speed. The properties of the coated wires are shown in Table I. TABLE I Al₂O₃ in dielectric elongation softening heat lifetime⁽²⁾ Ex. upper layer flexibility adherence (kV) (%) temp. (° C.) shock⁽¹⁾ (Hr) C1 none Good good 8.86 33.5 402 good 39.4 C2 α-Al₂O₃ Good good 8.69 32.5 407 good 23.8 C3 γ-Al₂O₃ Good good 9.53 32 419 good 41 C4 (α + γ)Al₂O₃ Good good 8.92 33 412 good 194.8 # 195° C. constant-temperature furnace.

[0034] As shown in Table I, the pulsed voltage surge lifetime of the enamelled wires of the present invention wherein the shield layer contains both α-form Al₂O₃ particles and γ-form Al₂O₃ particles is much higher than that of the enamelled wires wherein the shield layer contains α-form Al₂O₃ particles or γ-form Al₂O₃ particles.

Examples 1-4

[0035] The same as Comparative Example 1 with the exception that 1% Al₂O₃ particles (α-form/γ-form=1/9) and 0%, 5%, 10%, or 15% Cr₂O₃ particles, based on the weight of synthetic resin, were added to the PAI coating and the metal oxide particle-containing PAI coatings were mixed at high stirring speed. The properties of the coated wires are shown in Table II. TABLE II particles in dielectric elongation softening heat lifetime⁽³⁾ Ex. upper layer flexibility adherence (kV) (%) temp. (° C.) shock (Hr) 1 1%(α + γ)Al₂O₃ good good 8.92 33 412 good 2.1 2 1%(α + γ)Al₂O₃ + good good 9.2 33 395 good 6.1 5%Cr₂O₃ 3 1%(α + γ)Al₂O₃ + good good 9.1 32 376 fair 12.5 10%Cr₂O₃ 4 1%(α + γ)Al₂O₃ + fair fair 9.0 32 365 fair 27.2 15%Cr₂O₃

[0036] As shown in Table II, the pulsed voltage surge lifetime of the enamelled wires of the present invention wherein the shield layer contains α-form and γ-form Al₂O₃ particles and transition metal oxide particles, such as Cr₂O₃ particles, is much higher than that of the enamelled wires wherein the shield layer contains α-form Al₂O₃ particles and γ-form Al₂O₃ particles.

[0037] While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention. 

1. A pulsed voltage surges resistant enamelled wire comprising a metal conductive wire and at least one coating layer outside the wire, wherein at least one of the coating layer(s) is a shield layer containing metal oxide particles and the metal oxide particles comprise α-form and γ-form Al₂O₃ particles and transition metal oxide particles.
 2. The enamelled wire according to claim 1, wherein the shield layer is provided by a coating composition comprising (a) a synthetic resin, (b) an organic solvent and (c) metal oxide particles, wherein the metal oxide particles comprise α-form and γ-form Al₂O₃ particles and transition metal oxide particles.
 3. The enamelled wire according to claim 1, wherein the transition metal oxide is selected from the group consisting of Cr₂O₃, TiO₂, SiO₂, Zr₂O₃, ZnO and Fe₂O₃.
 4. The enamelled wire according to claim 3, wherein the transition metal oxide is Cr₂O₃.
 5. The enamelled wire according to claim 2, wherein the coating composition comprises from 3 to 20 parts by weight of metal oxide particles per hundred parts by weight of the synthetic resin.
 6. The enamelled wire according to claim 5, wherein the coating composition comprises from 5 to 15 parts by weight of metal oxide particles per hundred parts by weight of the synthetic resin.
 7. The enamelled wire according to claim 5, wherein the coating composition comprises from 2 to 19 parts by weight of transition metal oxide particles per hundred parts by weight of the synthetic resin.
 8. The enamelled wire according to claim 7, wherein the coating composition comprises from 5 to 15 parts by weight of transition metal oxide particles per hundred parts by weight of the synthetic resin.
 9. The enamelled wire according to claim 8, wherein the coating composition comprises from 5 to 10 parts by weight of transition metal oxide particles per hundred parts by weight of the synthetic resin.
 10. The enamelled wire according to claim 5, wherein the ratio between the α-form Al₂O₃ particles and the γ-form Al₂O₃ particles is from 1:1 to 1:100.
 11. The enamelled wire according to claim 5, wherein the ratio between the α-form Al₂O₃ particles and the γ-form Al₂O₃ particles is from 1:5 to 1:50.
 12. The enamelled wire according to claim 5, wherein the ratio between the α-form Al₂O₃ particles and the γ-form Al₂O₃ particles is from 1:5 to 1:15.
 13. The enamelled wire according to claim 1, wherein the particle size of the metal oxide particles is from 0.01 to 5 microns.
 14. The enamelled wire according to claim 2, wherein the synthetic resin is selected from the group consisting of modified or unmodified polyacetal, polyurethane, polyester, polyesterimine, polyimine, polyamide, polysulfone, polyimide resins and mixtures thereof.
 15. The enamelled wire according to claim 2, wherein the organic solvent is selected from the group consisting of cresols, hydrocarbons, dimethyl phenol, toluene, xylene, ethyl benzene, N,N-dimethyl formamide, N-methylpyrrolidone, esters, ketones, and mixtures thereof.
 16. The enamelled wire according to claim 2, wherein the coating composition comprises, based on the total weight of the synthetic resin and the organic solvent, from 80 to 20 wt % synthetic resin and from 20 to 80 wt % organic solvent.
 17. The enamelled wire according to claim 16, wherein the coating composition comprises, based on the total weight of the synthetic resin and the organic solvent, from 75 to 25 wt % synthetic resin and from 25 to 75 wt % organic solvent. 